lucy24 - 4:37 am on Oct 29, 2011 (gmt 0)
Most sites people go to are HTTP so it was decided to just not bother showing the protocol in newer versions of Firefox unless it's something else, like https:// or ftp://, thus saving a little space and making the URL a little more readable.
(note use of the passive voice)
You can set the pref browser.urlbar.trimURLs to false on the about:config page to see the http: protocol
In other words: if it shows nothing, it means it's the default http:
... at least until people start reporting exceptions.